1. Field of the Invention
The present invention relates to a frequency stabilized light source, and more particularly, relates to a frequency stabilized light source for generating frequencies spaced at predetermined intervals, such as local oscillation lights on the side of a receiver.
In coherent light communication, a frequency multiplexing may be utilized in which a number of signals having carrier frequencies are spaced at uniform intervals. In such communication system, in order to perform heterodyne (or homodyne) detection on the receiver side, a local oscillation light source having the same frequency intervals as those on the transmitter side is utilized.
The present invention provides a frequency stabilized light source having frequencies spaced at uniform intervals which can be used, for example, as a localized oscillation light source on the side of a receiver.
2. Prior Art
A construction of the conventional frequency stabilized light source will be first explained in reference to FIG. 5. In this figure, 1 is a reference signal source, 2 is a comparator, 5 is a light source and 6 is a frequency detector. The change in frequency of the light from the light source 5 is detected by the frequency detector 6 and is compared with the reference signal from the reference signal source 1. The comparator 2 detects the frequency deviation of the light source 3 which is fed back to the light source to stabilize it.
As the frequency detector 6, a Fabry-Perot interferometer or a gas absorption spectrometer is chiefly used. The curve 31 of FIG. 6 shows a detection profile of the Fabry-Perot interferometer and the curve 41 of FIG. 8 shows a detection profile of the gas spectrometer. In the former case, the point 33 on the profile of FIG. 6 is selected as a detection point and, in the latter case, the point 43 of FIG. 8 is selected as a detection point. However, the detection property of many frequency detectors 6 is obtained in a very restricted frequency range. For example, in the Fabry-Perot interference meter, only the range between the points 32 and 34 in FIG. 6 is available for a detector and in the gas spectrometer only the range between the points 42 and 44 in FIG. 8 is usable for a detector.
The frequency of the light source 5 in FIG. 5 can be varied by adjusting the light source 5 and the frequency detector 6. In the case of Fabry-Perot interference meter as the frequency detector 6, the detection property is adjusted to have a property as expressed by a curve 36 in FIG. 7 by controlling the conditions such as interference distance, refractive index, temperature and pressure, so that the point 33 in FIG. 6 is shifted to the point 37 in FIG. 7. In the case of gas spectrometer, the detection property is adjusted to have a detection property as expressed by the curve 46 in FIG. 9 so that the point 43 in FIG. 8 is shifted to the point 47 in FIG. 9.